Chiroptera

Bats

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Introduction

The clade Chiroptera includes two extant clades, Megachiroptera (Old World Fruit Bats) and Microchiroptera (echolocating bats). In addition, Chiroptera includes at least four extinct clades that are most closely related to Microchiroptera. There are over nine hundred extant species of bats (Koopman, 1993). Bats vary greatly in size. The smallest bat, Craseonycteris thonglongyai (Microchiroptera), weighs less than 2 g and has a wingspan of 12-13 cm, while the largest bats, those of the genus Pteropus (Megachiroptera), weigh up to 1.5 kg and may have a wing span over 2m (Fenton, 1992).

Bats are unique among mammals as they are the only group to have evolved true powered flight. Some other mammals such as "flying" squirrels and "flying" lemurs can glide through the air for long distances, but they are not capable of sustained flight. In contrast, bats can propel themselves with their wings, gaining and loosing altitude and flying for long periods.

Bats are nocturnal and usually spend the daylight hours roosting in caves, rock crevices, trees, or manmade structures such as houses and/or bridges. Some bats are solitary, while others are found in colonies that may include over a million individuals.

Activity begins around dusk, when bats leave the day roost and start feeding. The clade Chiroptera includes species with very diverse food preferences, including bats that eat either meat, insects, fish, fruit, nectar, or a variety of food types. Only three species of bats actually feed on blood Desmodontinae). Many bats remain at their feeding sites until just before dawn when they return to the day roost.

Characteristics

Unique Characteristics

One unique feature of bats is their modified forelimbs, which support a wing membrane (patagium). The basic elements of the mammalian limb are present in bats, although the relative sizes of most bones and muscles differ from those of nonflying mammals. The most elongated parts of the limb are those of the hand (metacarpal bones) and fingers (phalanges). The primary functions of these bones in bats is to provide support for the patagium and control its movements. The patagium stretches between the fingers and attaches to the side or back of the bat and the lower leg. Part of the membrane extends between the hindlimbs. Numerous blood vessels and nerves are present throughout the wing membrane. Bats also have five unique muscles present in the patagium, and use additional muscles in the chest and back to move the wings up and down.

The most obvious difference between bird wings and those of bats is that bird wings are made of feathers, not a skin membrane. Birds have an elongated arm, but do not have elongated fingers like bats. Additionally, the muscles used in both the upstroke and downstroke are found in the chest of birds, while the upstroke muscles are on the back in bats (Fenton, 1983).

The orientation of the hindlimb is also unique to bats. The hip joint is rotated 90° so that the legs project sideways and the knee faces almost backwards. Due in part to the rotation of the hindlimb, the walking motion of bats differs from other tetrapods, often appearing awkward. The hindlimb is designed to support the patagium in flight and allow the bat to roost hanging from its hindlimbs. Most bats have a tendon system in the toes that locks the claws in place so the bat can hang upside down even when asleep.

General Characteristics

The body of a bat is ventrally compressed with a short neck region. The bones tend to be slender and light-weight. The majority of the body weight is concentrated in the chest region due to the large flight muscles.

The overall shape of the head varies more in bats than within most other groups of mammals. Some bats have very elongated muzzles while others have broad, short faces. There is a correlation between the shape of the head and the type of food eaten. For example, most nectar feeders have long, narrow muzzles that are good for reaching into flowers, while many fruit eaters have short, broad faces good for biting rounded fruits (Hill and Smith, 1984).

The ears range from small and round to large and pointed, and often have a cartilaginous fold (tragus) present at the notch of the ear. There is additional variation in the nasal and lip regions of bats. Some bats have complex noseleafs, folds, or wrinkles on their muzzles. The function of facial ornamentation is not well understood, although it may effect the emission of echolocation calls in some taxa (Fenton, 1992).

A major misconception about bats is that they are blind. This idea originated from the fact that bats are able to successfully maneuver in the dark and often have small eyes. While some bats do have very small eyes (most Microchiroptera) many have large and complex eyes (Megachiroptera). Experiments on several species of bats have shown that they are able to distinguish patterns even at low light levels (Hill and Smith, 1984).

Bats usually have black or brown fur, although the fur can also be gray, white, red, or orange. In some species there are stripes on the face or down the back, or patches of white on the face or above the shoulder. The length of the fur also varies among species from short and dense to long and fluffy. The wing membrane is usually dark in color, although it may have white on the tips or be a lighter color around the bones in the membrane. A few bats have white or pale yellow wings. There are also little hairs on the membrane itself. These hairs can be the color of the wing or the same color as the body.

Bat Monophyly

Until the 1970s, most evolutionary biologists assumed that bats form a monophyletic group. Recently, however, several authors have questioned monophyly of Chiroptera (Jones and Genoways, 1970; Smith, 1976, 1980; Smith and Madkour, 1980; Hill and Smith, 1984; Pettigrew, 1986, 1991a, 1991b; Pettigrew and Jamieson, 1987; Pettigrew et al., 1989) creating what has become known as the “bat monophyly controversy”. Proponents of the hypothesis that bats are diphyletic pointed out that many similarities between Megachiroptera and Microchiroptera involve the flight mechanism. It is therefore possible that convergent evolution of aerial locomotion, rather than shared ancestry, might account for the similarities found between megachiropteran and microchiropteran bats (Jones and Genoways, 1970).

The bat monophyly hypothesis states the Megachiroptera and Microchiroptera are each others closest relatives in an evolutionary sense (i.e., they form a clade). If this is true, then their shared characteristics, including the ability to fly, would have been present in their most recent common ancestor (Simmons, 1994; 1995). It follows from this that there was only one origin of powered flight in mammals. In contrast, the diphyly hypothesis states that megachiropteran and microchiropteran bats do not form a monophyletic group, instead having evolved independently from two different groups of non-flying mammals. It has been suggested that Megachiroptera is more closely related to Dermoptera and Primates than to Microchiroptera (Smith and Madkour, 1980; Pettigrew, 1986, 1991a, b, 1995; Pettigrew and Jamieson, 1987; Pettigrew et. al., 1989). In this case, the characteristics common to both groups of bats either evolved as a result of convergent evolution or are simply the result of retention of primitive features. If bats are diphyletic, the ability to fly must have evolved once in Megachiroptera and again in Microchiroptera.

Because the vast majority of available data strongly support a sister-group relationship between Megachiroptera and Microchiroptera, bat monophyly is now regarded as a very strongly supported hypothesis.

Discussion of Phylogenetic Relationships

The fossil record of bats extends back at least to the early Eocene, and chiropteran fossils are known from all continents except Antarctica. Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx, unlike most other fossil bats, have not been referred to any extant family or superfamily. These Eocene taxa are known from exceptionally well-preserved fossils, and they have long formed a basis for reconstructing the early evolutionary history of Chiroptera (see review in Simmons and Geisler, 1998).

Smith (1977) suggested that these taxa represent an extinct clade of early microchiropterans ("Palaeochiropterygoidea"). In contrast, Van Valen (1979) argued that these fossil forms are representatives of a primitive grade ancestral to both Megachiroptera and Microchiroptera ("Eochiroptera"). Novacek (1987) reanalyzed morphology of Icaronycteris and Palaeochiropterx and concluded that they are more closely related to Microchroptera than to Megachiroptera. Most recently, Simmons and Geisler (1998) found that Icaronycteris, Archaeonycteris, Hassianycteris, and Palaeochiropteryx represent a series of consecutive sister-taxa to extant microchiropteran bats.

Geographic Distribution

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The geographical distribution of bats is shown in red. Distribution from Hill and Smith (1984).

Simmons, N. B. & J.H. Geisler. 1998. Phylogenetic relationships of Icaronycteris, Archeonycteris, Hassianycteris, and Palaeochiropteryx to extant bat lineages, with comments on the evolution of echolocation and foraging strategies in microchiroptera. Bulletin of the American Museum of Natural History. 235:1-182.

Page: Tree of Life
Chiroptera. Bats.
Authored by
Nancy B. Simmons and Tenley Conway.
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